MDF Face Vs Barrier Mat - Membrane Bass Trap Absorber

[Anna-Logue]

Hello,

I need to build (a lot) of Membrane/Diaphragmatic bass traps targeting various modal frequencies of my room. (24.3 Hz - 100hz)

If I get the answer to my question about the lowest mode (24.3 Hz) then I can apply the same thinking to everything above that.

I want to know if thick MDF can be used as substite for Barrier Mat as a face/membrane for a membrane bass trap - a trap centered around 24Hz that is 400mm/15ft deep would require a 18mm/0.7" thick MDF membrane (assuming a density of 12.8kg/m2)

My question: Is there a problem with using such a thick and rigid membrane - does the stiffness of it hamper its performance over something like a barrier mat? Is density the main factor or Rigidity and Density?

[Paulus87]

Hello,

I need to build (a lot) of Membrane/Diaphragmatic bass traps targeting various modal frequencies of my room. (24.3 Hz - 100hz)

If I get the answer to my question about the lowest mode (24.3 Hz) then I can apply the same thinking to everything above that.

I want to know if thick MDF can be used as substite for Barrier Mat as a face/membrane for a membrane bass trap - a trap centered around 24Hz that is 400mm/15ft deep would require a 18mm/0.7" thick MDF membrane (assuming a density of 12.8kg/m2)

My question: Is there a problem with using such a thick and rigid membrane - does the stiffness of it hamper its performance over something like a barrier mat? Is density the main factor or Rigidity and Density?

Hi,

You're comparing a limp membrane with a rigid panel absorber. Here's some good reading:

https://www.acousticsciences.com/art-no ... bass-traps

More excellent reading:

https://www.gearslutz.com/board/bass-tr ... traps.html

And this:

https://www.bbc.co.uk/rd/publications/rdreport_1992_10

And this:

https://ethanwiner.com/basstrap.html

Also this:

https://www.gearslutz.com/board/studio- ... lha-c.html

Targeting 24.5hz is very difficult to trap with a single device, unless you have a lot of space. I designed a membrane trap using 5kg/m2 MLV, 50mm air gap, 500mm of 5000rayls insulation (I call it "all the 5s'), it covers a theoretical range of 20Hz (0.6) - 50Hz (0.6) with a centre frequency of ~30Hz (0.8 ) but I have not tested it. It would also need to cover most of the rear wall.

Your walls may already be an effective membrane for the range of frequencies you're trying to target, if they are either lossy or a MAM system (Last gearslutz link).

Personally, I would be tempted to try some VPRs if you can afford it. You can read about them in the gearslutz link above. Very broad range of absorption and they only take up ~200mm.

MDF panels will resonate, but beware imperfect processing can cause differences in stated properties and actual e.g. surface density. If you decided to go this route then perhaps you could use some very sticky double sided foam tape to seal the front panel to the cabinet and use screws to "tune" it once it's installed. I think they might be expensive, risky and inefficient though compared to the other methods above.

Let us know how you get on as I also need to absorb some modal ringing down that low in my own space!

Paul

[DanDan]

Wow 24.3 Hz. I would reach for an Eq. Or some sort of cancellation thing with subs and delay.

Do you really have action down there?
+1 to Pauls thoughts. e.g. how about the whole wall. Perhaps floating the plasterboard on RC or Isoclips would create a massive Invisible Alpha © (Copyright Andre Vare)
VPR brings to mind the BBC wall design.... good to quite low https://www.bbc.co.uk/rd/publications/rdreport_1994_12

Speaking of, there are two variations on the market. One has very good absorption over a very useful band centred around 70Hz. 1mm Steel if I remember correctly.
The other is 2.5mm and is centred around 40Hz, but is much less efficient. So yea, bigger and heavier is harder to get moving.
These are both 100mm, so perhaps Paul's 200mm with say a 1.5mm steel...... who knows.

[Anna-Logue]

You're comparing a limp membrane with a rigid panel absorber. Here's some good reading:

Yes correct sorry, the types slipped my mind - thanks for the links they made some good reading

https://www.acousticsciences.com/art-no ... bass-traps

What stood out for me in this article is the adjusted formulas for Limp Vs Rigid Panel which takes the center frequency up by 1/3 for the rigid panel absorbers.

The adjustment for porous absorption which takes the center frequency down but widens the Q and reduces its efficiency.

The further adjustment for air volume - by adjusting the W X L by a small amount further reduces its center frequency.

Your walls may already be an effective membrane for the range of frequencies you're trying to target, if they are either lossy or a MAM system (Last gearslutz link).

This isn't possible for me as my walls are cement plastered brick.

Personally, I would be tempted to try some VPRs if you can afford it. You can read about them in the gearslutz link above. Very broad range of absorption and they only take up ~200mm.

Having looked into this it seems like a very viable option - I have come to the conclusion that too thick an MDF panel would probably be too rigid and ineffective. the steel would provide a much higher density at these lower thicknesses - I got some quotes today for 304 Grade Stainless Steel 1.5mm, 2mm, 3mm, 4.5mm - its not cheap, but it works out a similar price as the thicker MDF anyway, and certainly more cost effective, accessible, and denser than MLV. So everything is pointing in this direction for me right now. Is there a particular grade of Stainless Steel that is used? Also when it comes to the thickness of the steel I presume there would be an upper limit (wondering if 3mm is too thick?)

MDF panels will resonate, but beware imperfect processing can cause differences in stated properties and actual e.g. surface density. If you decided to go this route then perhaps you could use some very sticky double sided foam tape to seal the front panel to the cabinet and use screws to "tune" it once it's installed. I think they might be expensive, risky and inefficient though compared to the other methods above.

Yes this is true and would hold true for the VPR Steel Panel too - my work around is to get the pieces first, weigh them and then calculate the depth required based on measured weight/density.

The part that I am struggling the most with now is the design & calculation parameters because all of the calculators vary so vastly in their calculations?!

• A.) According to the calculator found at http://forum.studiotips.com/viewtopic.php?f=1&t=3497 (Membrane Absorbers.xlsx) - this has various options for membrane types (MDF, Steel, Gypsum etc - I don't see limp options):

270mm Deep, 25mm 703, 2mm Steel Panel Sheet (15.6kg/m2) = 24.36Hz (0.9 coef.)

• B.) Using the formulas at https://www.acousticsciences.com/art-no ... bass-traps

The results differ vastly except for the limp membrane with absorption. Based on these Results it Seems that the previous Calculator is working on Limp Membrane Calculation. (A similar Depth of 275mm yields the same result for the limp formula. Accounting for panel absorption using the adjusted formula yields the need for a 500mm deep trap with insulation.)

270mm Deep:
Limp Membrane:
f = 170 / (SQRT(3.19 * 10.63)) = 29.2 Hz (No porous Absorption)
f = 143 / (SQRT(3.19 * 10.63)) = 24.6 Hz (Formula Adjusted for porous absorption)
Rigid Panel:
f = 228 / (SQRT(3.19 * 10.63)) = 39.2 Hz (No porous Absorption)
f = 192 / (SQRT(3.19 * 10.63)) = 33.0 Hz (Formula Adjusted for porous absorption)

500mm Deep
Limp Membrane:
f = 143 / (SQRT(3.19 * 19.7)) = 18.0 Hz (Formula Adjusted for porous absorption)
Rigid Panel:
f = 192 / (SQRT(3.19 * 19.7)) = 24.2 Hz (Formula Adjusted for porous absorption)

• C.) Acoustic Modelling http://www.acousticmodelling.com/multi.php

This calculator only provides calculation for a limp membrane...

Using the initial spec: 270mm Deep, 25mm 703, 2mm Steel Panel Sheet (15.6kg/m2)

Provided: 28.6 Hz (0.05 Coef.) (basically useless)

To adjust the paramters to get 24.2 Hz (0.98 Coef.) I had to do 165 porous + 165 Air = 330mm

A.) says 270mm Deep
B.) says 500mm Deep
C.) Says 330mm Deep

Which do you use? Which do you choose? With such big variance between all the formula's and calculations how on earth is someone supposed to build these with fairly predictable results when they all say such different things?

It seems thats perhaps B.) is the most accurate because it adjusts for the Rigid Panel & The Insulation? and is seems that A.) is based on a limp membrane even though the selected option is "2mm Steel" and C.) explicity mentions "limp membrane"

[Anna-Logue]

Wow 24.3 Hz. I would reach for an Eq. Or some sort of cancellation thing with subs and delay.

Do you really have action down there?
+1 to Pauls thoughts. e.g. how about the whole wall. Perhaps floating the plasterboard on RC or Isoclips would create a massive Invisible Alpha © (Copyright Andre Vare)
VPR brings to mind the BBC wall design.... good to quite low https://www.bbc.co.uk/rd/publications/rdreport_1994_12

Speaking of, there are two variations on the market. One has very good absorption over a very useful band centred around 70Hz. 1mm Steel if I remember correctly.
The other is 2.5mm and is centred around 40Hz, but is much less efficient. So yea, bigger and heavier is harder to get moving.
These are both 100mm, so perhaps Paul's 200mm with say a 1.5mm steel...... who knows.

I want to rather treat the Funamentals and the problems with treatment than a EQ - I have tried EQing and it just doesnt sound right to me + the mode is still too distinguishable .... and irritating!

My walls are cement plastered brick - so Invisible Alpha © wont work for me unfortunately.

VPR seems the way to go - I am thinking to go with a 2mm Stainless Steel Sheet 304 grade.... it would just be good to find a reliable calculator for this?

I have read the BBC white paper many times - I have built MLV and MDF rigid panel absorbers based on this concept - but the adjusted formulas for the MDF rigid panel have probably turned my measurements upside down. The MLV Limp traps should be in the ball park of 50hz. MLV is too expensive to import for the scale that I need and MDF is too thick and rigid.

These are the modes I need to target at various positions - just need a reliable way to calculate the depth
24.3hz 1-0-0
36.39hz 0-1-0
50.7hz 2-0-0
74.2hz 0-2-0

[Paulus87]

It seems there is less “risk” when using steel compared to other materials since the panel has a large Q and covers a bigger range of frequencies. Apparently.

1.5mm steel seems to be the most reactive, I think I would go with that and make them 1.5m x 2m, and fit them the total length of the back wall, or two in each corner and one in the middle.

You could sandwich the steel inside a rockwool sandwich 100mm each side, stuff it all inside a 200mm frame and stretch some strong fabric across both sides to keep it all together, then mount the frame to the wall. This will leave the steel to resonate freely. The back side rockwool should be up against the wall and you can drill holes/cut slots in the frame to increase absorption on the sides.

If you want to get really fancy then a twist on this is John Brandt’s design on his website, he suspends the steel in a rubber envelope which is held in place by a metal frame. The frame is then secured to the cabinet and can be moved further out/back in order to tune the frequency further if needed. Tedious but snazzy.

Regarding the calculators - I’m as clueless as you. I use the simple multi layer porous absorber calculator for quick dirty experiments with velocity traps, and then the manual formulas in the article I linked you to for membrane traps. I have to admit most of the time I am just running on intuition and trial and error.

Let us know how you get on!

[DanDan]

Soundflow seems to be a well regarded commercial app.
I would confidently guess that Paul's 200mm arrangement would work fabulously well. But note Membrane or VPR will primarily absorb only on the axis they address. So straddling corners would seem wise.
I suggest a more holistic approach here. What is the size of the space? What are the floor and ceiling made of?

[Anna-Logue]

It seems there is less “risk” when using steel compared to other materials since the panel has a large Q and covers a bigger range of frequencies. Apparently.

1.5mm steel seems to be the most reactive, I think I would go with that and make them 1.5m x 2m, and fit them the total length of the back wall, or two in each corner and one in the middle.

You could sandwich the steel inside a rockwool sandwich 100mm each side, stuff it all inside a 200mm frame and stretch some strong fabric across both sides to keep it all together, then mount the frame to the wall. This will leave the steel to resonate freely. The back side rockwool should be up against the wall and you can drill holes/cut slots in the frame to increase absorption on the sides.

If you want to get really fancy then a twist on this is John Brandt’s design on his website, he suspends the steel in a rubber envelope which is held in place by a metal frame. The frame is then secured to the cabinet and can be moved further out/back in order to tune the frequency further if needed. Tedious but snazzy.

Regarding the calculators - I’m as clueless as you. I use the simple multi layer porous absorber calculator for quick dirty experiments with velocity traps, and then the manual formulas in the article I linked you to for membrane traps. I have to admit most of the time I am just running on intuition and trial and error.

Let us know how you get on!

Thanks for the insight and suggestions Paul - it makes more sense now that I have done a bit more research on VPR/CBA absorbers now - My understanding was completely wrong - although I think it is still possible to build a sealed tuned device using a steel plate like the Rigid Panel Design but this isnt stricly a VPR device. See my further comments on VPR/CBA below...

Soundflow seems to be a well regarded commercial app.
I would confidently guess that Paul's 200mm arrangement would work fabulously well. But note Membrane or VPR will primarily absorb only on the axis they address. So straddling corners would seem wise.
I suggest a more holistic approach here. What is the size of the space? What are the floor and ceiling made of?

DanDan - I am so glad you commented on my thread lol - I have seen your name a lot in the past 2 days while delving deep into the mystery of the VPR device and a lot of my summary notes have come from you, so this is encouraging, thank you! SoundFlow looks like an interesting program, I will definitely download it and see what it comes up with and how it compares to other models.

Floor is tiled, wall is cement plastered brick, the ceilings are 2x15mm (30mm) dry wall sealed and skimmed over. The dimensions:

6,700mm (29ft) L x 4,585mm (15ft) W x 3,675mm (12.05ft) H
(the ceiling is vaulted so this is from the floor to the pinnacle of the ceiling) the average height from floor to ceiling is 3.13m (10.27ft)

room volume: 96.2 m3 / 3397 ft3 (using average ceiling height)
Room Area: 30.7m2 / 330.6 ft²

I have attached plans, layouts and pictures of the room (WIP), and the mdat file of the room in its current state - the arbitrary panel absorbers are in "test" locations because I cant seen to figure out where they will work best - buts its probably more because I don't have enough of the right type. the pictures are attached to the pdf - I am struggling to upload the mdat file - this is here: https://www.sendspace.com/filegroup/rqm ... G8yyQQ11HQ

Upon reading Paul's suggestion of the VPR absorber I have spent the past 2 days going on a MISSION to try find out as much as I can about it my initial idea of it was completely wrong. I see this as being a possible solution to my modal problems while keeping the depth reasonable.

I have collected and read a bunch of research papers, calcluators, datasheets, and read a lot of threads. its a lot to wrap your head around and sometimes it gets confusing and I am left with uncertainty and more questions than answers about certain aspects. some key points for me are - does this thing absorb effectively/effiencienctly down to 20Hz? and is the 2.5mm plate better for < 50Hz than the 1mm plate. The CPA-A2 seems to suggest so but the comments in threads not so much. The Foam can be CIB or Basotect but I have read that CIB is a better performer - although BASF is in my country - But I need to find out if its something they have here in which case I would rather import CIB - I am also uncertain about the thickness and density the CBA-A2 uses 100mm 40kg/m2 but I see other much lower densities being used - this affects the center freq? Also Paul suggests that normal insulation can be used instead of foam, or is foam better, if insulation can be used which density this this affect freq? Mounting/glueing the plate so that is is free standing also poses many questions for me as to how you actually mount the unit without the foam bending down - or is the foam rigid enough and gluing the back side of the foam to the wall keeps it up? I have tried using the caluclator xls to get the plate modes as close to my room modes as possible - when seeing a mode like 24/25hz - does this mean it will be effective here - not sure how to interpret it, because the center frequency still seems to lie > 100Hz

I'm kind of at the point about knowing what to do - but not how.

I understand I need a lot more trapping at many of the modal presures

24.3hz 1-0-0
36.39hz 0-1-0
50.7hz 2-0-0
74.2hz 0-2-0

And using Amroc and a SPL meter I can see where these are - but im stuck on the how do I go about building different units to treat these using all the parameters that go into building them

[Paulus87]

It seems there is less “risk” when using steel compared to other materials since the panel has a large Q and covers a bigger range of frequencies. Apparently.

1.5mm steel seems to be the most reactive, I think I would go with that and make them 1.5m x 2m, and fit them the total length of the back wall, or two in each corner and one in the middle.

You could sandwich the steel inside a rockwool sandwich 100mm each side, stuff it all inside a 200mm frame and stretch some strong fabric across both sides to keep it all together, then mount the frame to the wall. This will leave the steel to resonate freely. The back side rockwool should be up against the wall and you can drill holes/cut slots in the frame to increase absorption on the sides.

If you want to get really fancy then a twist on this is John Brandt’s design on his website, he suspends the steel in a rubber envelope which is held in place by a metal frame. The frame is then secured to the cabinet and can be moved further out/back in order to tune the frequency further if needed. Tedious but snazzy.

Regarding the calculators - I’m as clueless as you. I use the simple multi layer porous absorber calculator for quick dirty experiments with velocity traps, and then the manual formulas in the article I linked you to for membrane traps. I have to admit most of the time I am just running on intuition and trial and error.

Let us know how you get on!

Thanks for the insight and suggestions Paul - it makes more sense now that I have done a bit more research on VPR/CBA absorbers now - My understanding was completely wrong - although I think it is still possible to build a sealed tuned device using a steel plate like the Rigid Panel Design but this isnt stricly a VPR device. See my further comments on VPR/CBA below...

Soundflow seems to be a well regarded commercial app.
I would confidently guess that Paul's 200mm arrangement would work fabulously well. But note Membrane or VPR will primarily absorb only on the axis they address. So straddling corners would seem wise.
I suggest a more holistic approach here. What is the size of the space? What are the floor and ceiling made of?

DanDan - I am so glad you commented on my thread lol - I have seen your name a lot in the past 2 days while delving deep into the mystery of the VPR device and a lot of my summary notes have come from you, so this is encouraging, thank you! SoundFlow looks like an interesting program, I will definitely download it and see what it comes up with and how it compares to other models.

Floor is tiled, wall is cement plastered brick, the ceilings are 2x15mm (30mm) dry wall sealed and skimmed over. The dimensions:

6,700mm (29ft) L x 4,585mm (15ft) W x 3,675mm (12.05ft) H
(the ceiling is vaulted so this is from the floor to the pinnacle of the ceiling) the average height from floor to ceiling is 3.13m (10.27ft)

room volume: 96.2 m3 / 3397 ft3 (using average ceiling height)
Room Area: 30.7m2 / 330.6 ft²

I have attached plans, layouts and pictures of the room (WIP), and the mdat file of the room in its current state - the arbitrary panel absorbers are in "test" locations because I cant seen to figure out where they will work best - buts its probably more because I don't have enough of the right type. the pictures are attached to the pdf - I am struggling to upload the mdat file - this is here: https://www.sendspace.com/file/muygi4

Upon reading Paul's suggestion of the VPR absorber I have spent the past 2 days going on a MISSION to try find out as much as I can about it my initial idea of it was completely wrong. I see this as being a possible solution to my modal problems while keeping the depth reasonable.

I have collected and read a bunch of research papers, calcluators, datasheets, and read a lot of threads. its a lot to wrap your head around and sometimes it gets confusing and I am left with uncertainty and more questions than answers about certain aspects. some key points for me are - does this thing absorb effectively/effiencienctly down to 20Hz? and is the 2.5mm plate better for < 50Hz than the 1mm plate. The CPA-A2 seems to suggest so but the comments in threads not so much. The Foam can be CIB or Basotect but I have read that CIB is a better performer - although BASF is in my country - But I need to find out if its something they have here in which case I would rather import CIB - I am also uncertain about the thickness and density the CBA-A2 uses 100mm 40kg/m2 but I see other much lower densities being used - this affects the center freq? Also Paul suggests that normal insulation can be used instead of foam, or is foam better, if insulation can be used which density this this affect freq? Mounting/glueing the plate so that is is free standing also poses many questions for me as to how you actually mount the unit without the foam bending down - or is the foam rigid enough and gluing the back side of the foam to the wall keeps it up? I have tried using the caluclator xls to get the plate modes as close to my room modes as possible - when seeing a mode like 24/25hz - does this mean it will be effective here - not sure how to interpret it, because the center frequency still seems to lie > 100Hz

I'm kind of at the point about knowing what to do - but not how.

I understand I need a lot more trapping at many of the modal presures

24.3hz 1-0-0
36.39hz 0-1-0
50.7hz 2-0-0
74.2hz 0-2-0

And using Amroc and a SPL meter I can see where these are - but im stuck on the how do I go about building different units to treat these using all the parameters that go into building them

The foam or insulation is just to damp the steel panel, I wouldn't be too concerned with the density or GFR or this layer since IMO it's not going to make much difference. The front layer is so that the steel panel does not become a reflector and you will gain some more broadband absorption by using it. I suggested Rockwool because it is usually easy to get hold of unlike basotec etc. and it has been proven to work very effectively. Basotec is a nice product for many reasons, but it's more expensive and harder to find and I don't think the actual acoustic properties of it will be much different to insulation in this particular application - Dan might disagree?

In regards to holding it all together and mounting it - as I said, you could fit it all within a timber frame, put some braces on the front and back to keep the insulation/foam in place and the insulation/foam will keep the steel in place. Then you can mount the timber frame to the wall and/or build a low level shelf to support it if not being place on the floor.

Have you actually measured your room? Or are you just going purely on what the calculator tells you?

How much you need to treat the calculated modes, and how accurate the calculation is will be confirmed by measuring the room. You need to see which frequencies are problematic and how long their decay is which will help you know what you actually need to treat. The calculator can help you know where those pressure zones are, but the calculator should not be the basis for your treatment plan - Until you measure your room (I'm sorry if you already have, but you haven't mentioned it or shared any REW data etc) there's no point in even talking about membrane traps etc because the calculator will only estimate what you can expect if your room was built using perfect materials, mm perfect dimensions, perfectly rectangular with very heavy solid rigid boundaries with no inconsistencies or imperfections. And even then, you'd have to build your walls so that they perfectly isolate sub 20hz, do not absorb any sound and are completely reflective.

And then there's the fact that 24.3hz is rarely triggered when actually listening to/mixing most music, so is it really a big deal if it's not completely treated? Are you mixing sound FX or stupidly fat hip hop kick drums? or pipe organs?

Paul

[DanDan]

You are welcome Anna. I have tended to gush about devices and techniques as I discovered them over the years.
Blackbird Studio 3 I think, designed by D'Antonio and Massenburg, Gods, has something like 38 VPRs......
But back to your room, a very solid and decent sized space. It may well have good modal density.
Measure it acoustically, and +1 to everything else that Paul said.
To add a little.... I doubt that the fibre is critical in any way, BUT you can buy exactly any of those materials easily now. https://www.don-audio.com/acoustic-absorbers
Looking at the published data by RPG, Renz, Fraunhofer etc. the 2.5mm steel is good around 40Hz but not a very efficient absorber.
In any case, remember these absorption figures are sort of qualitative. A lot of area is needed to get anything done at 40Hz.
One could try 1.5 mm on a pad of 150 or 200mm deep. Add 50-100mm on the room side to deal with HF/MF. I wouldn't sweat the mounting or glueing arrangements. A simple wooden frame to keep the sandwich together perhaps. Fraunhofer used double sides sticky tape. Gernot seems to have just pushed the plate against the fibre.
I have an ambition to try one here in an alcove at the back of my CR. My plan is to hang the huge heavy sheet by chains from the ceiling beams above. But I would attack the top of the chains a bit closer to the wall than the thickness of CIB. Thus the hanging pressure should keep the plate pressing inward. I hope! Does this make sense?

Can we skip the 24Hz for the sake of progressing, whatever broadband we do will address it a little bit in any case.

I am presuming this is a Mix or Mixed use room. Probably the single most powerful intervention would be treating the whole back wall with cheap light fibre. If you can source the low density batts, say 16 or 24KG they are easy to work with. Simple frame 60cm trapping.
Look up Bogic Petrovic on Instagram or on FaceCloth. A fabulous man, sadly missed.
If you are prepared to lose width, do the same at the sides. Decent depth and area of Cloud, done.

[Anna-Logue]

Its good to know that these VPR-esque type traps are possible using regular insulation and they are not so finicky as one would be lead to believe reading all of the very early documentation - it seems the BBC has done a similar type device using a steel plate and rockwool so thats encouraging.

My initial room measurements were attached to my previous posts I think you guys might have missed them - includes pics I was having trouble uploading the Mdat file so I had to use a file host.

I was relying on someone else to do my room measurements for me using his Mic, SPL meter and note book - I bought a Dayton Audio EMM-6 Mic and a SPL meter which arrived this week - so I did some measurements of my room last night of the L, R speakers separately and I had the time to do the calibration and gain staging correctly - Im still having trouble uploading the mdat files so they are here on file host:https://www.sendspace.com/filegroup/rqm ... G8yyQQ11HQ

the limp membrane traps are all in the corners so i think this has reduced the 49/50Hz mode some what but its still hanging around there. the 54Hz 0-0-1 mode is still very long - which explains why I keep hear it coming from the ceiling when those sustained notes are played!

I put some layered trapping paramaters into SoundFlow and Acoustic modelling and these are the results I get. Im not sure if the paramaters are so out of whack that actually building it would behave like this?

Parameters:
Total Depth: 918mm
Model: Miki
Dimension 1.2m W x 2.4m H
Layer 1: Rigid Backing.
Layer 2: Absorber: 50mm, 48 kg/m3, 16000 Pa.s/m2.
Layer 3: 700mm Air
Layer 4: Perforated Panel: 18mm MDF - hole diameter (20mm, hole spacing 20mm.(78.54%) / (Slit 20mm & Slot Spacing 20mm (100%) - the perforated (pink) and slat (red) are similar - slat seems to go down a bit further.
Layer 5: Absorber: 50mm, 48 kg/m3, 16000 Pa.s/m2.

[Paulus87]

Its good to know that these VPR-esque type traps are possible using regular insulation and they are not so finicky as one would be lead to believe reading all of the very early documentation - it seems the BBC has done a similar type device using a steel plate and rockwool so thats encouraging.

My initial room measurements were attached to my previous posts I think you guys might have missed them - includes pics I was having trouble uploading the Mdat file so I had to use a file host.

I was relying on someone else to do my room measurements for me using his Mic, SPL meter and note book - I bought a Dayton Audio EMM-6 Mic and a SPL meter which arrived this week - so I did some measurements of my room last night of the L, R speakers separately and I had the time to do the calibration and gain staging correctly - Im still having trouble uploading the mdat files so they are here on file host: https://www.sendspace.com/file/h34t7f

the limp membrane traps are all in the corners so i think this has reduced the 49/50Hz mode some what but its still hanging around there. the 54Hz 0-0-1 mode is still very long - which explains why I keep hear it coming from the ceiling when those sustained notes are played!

I put some layered trapping paramaters into SoundFlow and Acoustic modelling and these are the results I get. Im not sure if the paramaters are so out of whack that actually building it would behave like this?

Parameters:
Total Depth: 918mm
Model: Miki
Dimension 1.2m W x 2.4m H
Layer 1: Rigid Backing.
Layer 2: Absorber: 50mm, 48 kg/m3, 16000 Pa.s/m2.
Layer 3: 700mm Air
Layer 4: Perforated Panel: 18mm MDF - hole diameter (20mm, hole spacing 20mm.(78.54%) / (Slit 20mm & Slot Spacing 20mm (100%) - the perforated (pink) and slat (red) are similar - slat seems to go down a bit further.
Layer 5: Absorber: 50mm, 48 kg/m3, 16000 Pa.s/m2.

Regardless of whether it would work or not, do you really want to build a trap almost 1m thick? That’s enormous for a membrane trap. Or is this just simply an exercise in demonstrating a hypothetical idea in sound flow?

The problem with mdf, plywood, osb etc is that they aren’t very reactive due to how rigid they are. That’s the beauty of the steel plate, it’s basically like a big cymbal, very easy to excite and resonant. With the damping this will eat up low frequency energy and it will do it in a fraction of the space of your proposed trap above.

The bbc design is excellent, I would like to build some very similar traps like those.

I had a look at prices for 1.5mm steel, 1m x 2m and it was between £60-£80 depending on who you buy it from. A sheet of thick mdf or ply would cost about half that, but they will take up more floor space.

The other thing to be aware of is membrane traps, especially if you have a lot of them and they’re big, heavy rigid things, will change your modal response due to the boundaries and room volume
Changing, so it’s a bit like playing cat and mouse. As soon as you think you’ve sorted your problem you may introduce new ones as a result of your room modes changing because of the large membranes.

Paul

[Anna-Logue]

Its good to know that these VPR-esque type traps are possible using regular insulation and they are not so finicky as one would be lead to believe reading all of the very early documentation - it seems the BBC has done a similar type device using a steel plate and rockwool so thats encouraging.

My initial room measurements were attached to my previous posts I think you guys might have missed them - includes pics I was having trouble uploading the Mdat file so I had to use a file host.

I was relying on someone else to do my room measurements for me using his Mic, SPL meter and note book - I bought a Dayton Audio EMM-6 Mic and a SPL meter which arrived this week - so I did some measurements of my room last night of the L, R speakers separately and I had the time to do the calibration and gain staging correctly - Im still having trouble uploading the mdat files so they are here on file host: https://www.sendspace.com/file/h34t7f

the limp membrane traps are all in the corners so i think this has reduced the 49/50Hz mode some what but its still hanging around there. the 54Hz 0-0-1 mode is still very long - which explains why I keep hear it coming from the ceiling when those sustained notes are played!

I put some layered trapping paramaters into SoundFlow and Acoustic modelling and these are the results I get. Im not sure if the paramaters are so out of whack that actually building it would behave like this?

Parameters:
Total Depth: 918mm
Model: Miki
Dimension 1.2m W x 2.4m H
Layer 1: Rigid Backing.
Layer 2: Absorber: 50mm, 48 kg/m3, 16000 Pa.s/m2.
Layer 3: 700mm Air
Layer 4: Perforated Panel: 18mm MDF - hole diameter (20mm, hole spacing 20mm.(78.54%) / (Slit 20mm & Slot Spacing 20mm (100%) - the perforated (pink) and slat (red) are similar - slat seems to go down a bit further.
Layer 5: Absorber: 50mm, 48 kg/m3, 16000 Pa.s/m2.

Regardless of whether it would work or not, do you really want to build a trap almost 1m thick? That’s enormous for a membrane trap. Or is this just simply an exercise in demonstrating a hypothetical idea in sound flow?

The problem with mdf, plywood, osb etc is that they aren’t very reactive due to how rigid they are. That’s the beauty of the steel plate, it’s basically like a big cymbal, very easy to excite and resonant. With the damping this will eat up low frequency energy and it will do it in a fraction of the space of your proposed trap above.

The bbc design is excellent, I would like to build some very similar traps like those.

I had a look at prices for 1.5mm steel, 1m x 2m and it was between £60-£80 depending on who you buy it from. A sheet of thick mdf or ply would cost about half that, but they will take up more floor space.

The other thing to be aware of is membrane traps, especially if you have a lot of them and they’re big, heavy rigid things, will change your modal response due to the boundaries and room volume
Changing, so it’s a bit like playing cat and mouse. As soon as you think you’ve sorted your problem you may introduce new ones as a result of your room modes changing because of the large membranes.

Paul

Hi Paul,

I think you missed the part that it is a Perforated (Pink line) and Slat (red line) Helmholtz Rezonator - but this could be done with wood for the slats (MDF for perforated board) This depth is for the 24/25Hz 1-0-0 mode so it is the back and front walls - the room is 6.7m long so I am happy to loose 2m to properly treat all the modes down to the lowest fundamental - this is also aimed to be broad band. the side wall modes (37Hz) I should be able to get away with a shallower trap.

Galvanized sheet steel is fairly cheap here - actually cheaper than MDF - they cut to size so you only pay for what you take - they don't charge for the off cuts which is great.

[DanDan]

Given your willingness to use 2M, I am quite sure you can achieve professional standard modal control using fibre absorption alone.

[Anna-Logue]

Given your willingness to use 2M, I am quite sure you can achieve professional standard modal control using fibre absorption alone.

Im open to all options (keep in mind this is 1m each side so effectively 2m in total) tell me more!...